There are at least two ways of looking at what Senate Republicans are trying to do this cycle. One is simple math: Pick up six seats and win back the majority in the upper chamber. More broadly, they are trying to snatch the Senate from the party that also controls the White House in the president's second midterm election.

It's worth looking back through history at the years in which the latter has happened, for clues about how feasible it may be in 2014. Since World War II, it's happened twice: 2006 and 1986.

In the 2006 Democratic wave election, George W. Bush and the Republican Party lost six Senate seats and 30 House seats, surrendering control of both chambers. Bush was broadly unpopular, with the public increasingly turning against the war in Iraq. The federal government's response to Hurricane Katrina, which came under heavy criticism, didn't help Bush, either.

In 1986, Ronald Reagan lost eight seats as Democrats took control of the Senate, which they last held in 1980. (The American Presidency Project at the University of California at Santa Barbara has a great chart that shows the losses presidents have sustained in second midterm elections.)

Six seats is no small task, but Republicans have plenty of targets in GOP-leaning states. As we've noted before, presidents haven't tended to have much luck in their second midterm election. The question for Democrats next is whether the macro-climate will be rough enough to trigger a wide shift in the Senate. It's more than conceivable that Democrats will lose some seats -- but enough to lose the majority? That's what we'll all be watching.

And now, to the Line! Below is our latest rundown of the top 10 Senate races of the cycle, ranked as usual from the safest — No. 10 — to the most in danger — No. 1.

10. Michigan (Democratic-controlled): Michigan comes on to the line following Democratic Sen. Carl Levin's decision not to run for reelection. Levin would have cruised to victory, and his retirement ever so slightly opens the door for Republicans. Democrats are still favored though; the GOP hasn't won a Senate race here since Spencer Abraham was elected in 1994. Neither party has a particularly deep bench in the state, notable considering Michigan's large population. Rep. Gary Peters (D) is broadly seen as the best Democratic option, while on the Republican side, a crowded race could be in the offing. There's talk that Rep. Mike Rogers (R) could run, but it's hard to see him giving up a plum gig as House Intelligence Committee chairman. If libertarian-leaning Rep. Justin Amash (R) makes a bid, he could give Republicans headaches. (Previous ranking: N/A)

9. Montana (D): The questions continue to be which Republican will emerge as a challenger to Sen. Max Baucus (D) and whether former governor Brian Schweitzer will primary him. If a top Republican emerges and/or Schweitzer runs, Baucus's life won't be too fun for the next year and a half. But until it is clear that one or both of those things will happen, there are a lot of other incumbents on this list who look much more vulnerable. (Previous ranking: 8)

8. Kentucky (Republican-controlled): Will she or won't she? Whether or not Ashley Judd (D) runs will dominate the discussion in this race until she announces a decision. National Democrats haven't publicly committed to backing Judd, and may end up preferring Secretary of State Alison Lundergan Grimes (D). Judd would be a fundraising behemoth, but the ads against her pratically write themselves (using "winter" as a verb is never a good thing for a candidate). As the Democratic picture gets sorted out, this much is clear: Senate Minority Leader Mitch McConnell (R) is very unpopular. And his decision to go up on the air so early in the cycle is a reflection of his vulnerable standing. (Previous ranking: 9)

7. Iowa (D): Republicans' chances of picking up this open seat received a blow when Rep. Tom Latham (R) removed himself from the list of possibilities. All eyes are on Rep. Steve King (R) now. If King runs, he would be hard to beat in a primary. But in a general election, King's outspoken brand of conservatism would be a vulnerability. Meanwhile, Democrats got the man they wanted when Rep. Bruce Braley (D) jumped in the race. (Previous ranking: 7)

6. Arkansas (D):The Club For Growth recently hit the airwaves with a major ad buy against Sen. Mark Pryor (D), stoking speculation over whether it was an effort to woo freshman Rep. Tom Cotton (R) to run. For his part, Cotton since he won't consider a Senate bid "for many months." Regardless, Pryor remains vulnerable in this increasingly Republican state. (Previous ranking: 6)

5. Louisiana (D): Whoever challenges Sen. Mary Landrieu (D), it won't be Rep. Charles Boustany (R), who recently took himself out of the running, or Lt. Gov. Jay Dardenne (R), who announced this week that he won't make a bid. (Boustany is a close ally of House Speaker John Boehner (R-Ohio), it's worth noting.) Other GOP names to keep an eye on include Rep. Bill Cassidy, Rep. John Fleming and former congressman Jeff Landry. (Previous ranking: 5)

4. North Carolina (D): Sen. Kay Hagan (D) landed a top operative this week when she announced Preston Elliot would run her reelection bid. Elliot guided Sen. Jon Tester (D) to victory against the odds in GOP-leaning Montana last cycle. In North Carolina, which has been trending increasingly Republican in recent elections, he'll need to defy a GOP lean once again. (Previous ranking: 4)

3. Alaska (D): Here's another race where the Democratic incumbent's outlook could be brightened or darkened depending on who the GOP nominee is. Lt. Gov. Mead Treadwell (R) is exploring a bid, and would be an attractive GOP candidate But if 2010 nominee Joe Miller (R) takes the plunge, it would be bad news for his party. (Previous ranking: 3)

2. South Dakota (D): Sen. Tim Johnson (D) might retire, and if he does, his son, U.S. Attorney Brendan Johnson, may step up. Another Democratic possibility is former congresswoman Stephanie Herseth Sandlin. Republicans have already landed a top recruit, former governor Mike Rounds (R), who would be a formidable nominee. The key now is whether he gets any kind of a primary challenge from his right or simply cruises to the party nod. (Previous ranking: 2)

1. West Virginia (D): The wait continues to see which Democrat(s) will step up in this race. So far, it's been smooth sailing for Rep. Shelley Moore Capito (R), who looks well-positioned for a pickup. It's very early, and a lot can happen between now and November 2014, but we'll need to see some Democrats emerge before we move this race away from the GOP. (Previous ranking: 1)

Sean Sullivan has covered national politics for The Washington Post since 2012.

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